X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FTarget%2FREADME.txt;h=341c5c1ff472f9f683fbc87384b4a44b9b0a2bc1;hb=0a0e68a7eac0513505aff3079e2d5d6864e51895;hp=b0d93f0264a3cff3fe1250a17595d57977911dd4;hpb=4e16b29a6b1830b977ac27de5954c1b9ee17984f;p=oota-llvm.git diff --git a/lib/Target/README.txt b/lib/Target/README.txt index b0d93f0264a..341c5c1ff47 100644 --- a/lib/Target/README.txt +++ b/lib/Target/README.txt @@ -2,13 +2,6 @@ Target Independent Opportunities: //===---------------------------------------------------------------------===// -We should make the various target's "IMPLICIT_DEF" instructions be a single -target-independent opcode like TargetInstrInfo::INLINEASM. This would allow -us to eliminate the TargetInstrDesc::isImplicitDef() method, and would allow -us to avoid having to define this for every target for every register class. - -//===---------------------------------------------------------------------===// - With the recent changes to make the implicit def/use set explicit in machineinstrs, we should change the target descriptions for 'call' instructions so that the .td files don't list all the call-clobbered registers as implicit @@ -30,7 +23,10 @@ Make the PPC branch selector target independant //===---------------------------------------------------------------------===// Get the C front-end to expand hypot(x,y) -> llvm.sqrt(x*x+y*y) when errno and -precision don't matter (ffastmath). Misc/mandel will like this. :) +precision don't matter (ffastmath). Misc/mandel will like this. :) This isn't +safe in general, even on darwin. See the libm implementation of hypot for +examples (which special case when x/y are exactly zero to get signed zeros etc +right). //===---------------------------------------------------------------------===// @@ -166,6 +162,9 @@ Expand these to calls of sin/cos and stores: Doing so could allow SROA of the destination pointers. See also: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17687 +This is now easily doable with MRVs. We could even make an intrinsic for this +if anyone cared enough about sincos. + //===---------------------------------------------------------------------===// Scalar Repl cannot currently promote this testcase to 'ret long cst': @@ -511,6 +510,8 @@ int i; } } +BasicAA also doesn't do this for add. It needs to know that &A[i+1] != &A[i]. + //===---------------------------------------------------------------------===// We should investigate an instruction sinking pass. Consider this silly @@ -925,35 +926,6 @@ vec2d foo () { //===---------------------------------------------------------------------===// -This C++ file: -void g(); struct A { int n; int m; A& operator++(void) { ++n; if (n == m) g(); -return *this; } A() : n(0), m(0) { } friend bool operator!=(A const& a1, -A const& a2) { return a1.n != a2.n; } }; void testfunction(A& iter) { A const -end; while (iter != end) ++iter; } - -Compiles down to: - -bb: ; preds = %bb3.backedge, %bb.nph - %.rle = phi i32 [ %1, %bb.nph ], [ %7, %bb3.backedge ] ; [#uses=1] - %4 = add i32 %.rle, 1 ; [#uses=2] - store i32 %4, i32* %0, align 4 - %5 = load i32* %3, align 4 ; [#uses=1] - %6 = icmp eq i32 %4, %5 ; [#uses=1] - br i1 %6, label %bb1, label %bb3.backedge - -bb1: ; preds = %bb - tail call void @_Z1gv() - br label %bb3.backedge - -bb3.backedge: ; preds = %bb, %bb1 - %7 = load i32* %0, align 4 ; [#uses=2] - - -The %7 load is partially redundant with the store of %4 to %0, GVN's PRE -should remove it, but it doesn't apply to memory objects. - -//===---------------------------------------------------------------------===// - Better mod/ref analysis for scanf would allow us to eliminate the vtable and a bunch of other stuff from this example (see PR1604): @@ -1086,16 +1058,6 @@ produces better code on X86. //===---------------------------------------------------------------------===// -From GCC Bug 33512: -int f(int y, int x) -{ - return x & ((~x) | y); -} -Should combine to x & y. Currently not optimized with "clang --emit-llvm-bc | opt -std-compile-opts". - -//===---------------------------------------------------------------------===// - From GCC Bug 15784: #define abs(x) x>0?x:-x int f(int x, int y) @@ -1157,12 +1119,6 @@ There's an unnecessary zext in the generated code with "clang //===---------------------------------------------------------------------===// -int a(int a, int b) {return ((a|b)&1) | (b&-2);} -Should be combined to "(a&1)|b". Currently not optimized with "clang --emit-llvm-bc | opt -std-compile-opts". - -//===---------------------------------------------------------------------===// - int a(unsigned b) {return ((b << 31) | (b << 30)) >> 31;} Should be combined to "((b >> 1) | b) & 1". Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". @@ -1181,12 +1137,6 @@ Should combine to "a | 1". Currently not optimized with "clang //===---------------------------------------------------------------------===// -int a(int a, int b, int c) {return ((a&~b) | (~a&b));} -Should fold to xor. Currently not optimized with "clang -emit-llvm-bc -| opt -std-compile-opts". - -//===---------------------------------------------------------------------===// - int a(int a, int b, int c) {return (~a & c) | ((c|a) & b);} Should fold to "(~a & c) | (a & b)". Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". @@ -1264,3 +1214,510 @@ Should combine to "20 * (((unsigned)x) & -2)". Currently not optimized with "clang -emit-llvm-bc | opt -std-compile-opts". //===---------------------------------------------------------------------===// + +We would like to do the following transform in the instcombiner: + + -X/C -> X/-C + +However, this isn't valid if (-X) overflows. We can implement this when we +have the concept of a "C signed subtraction" operator that which is undefined +on overflow. + +//===---------------------------------------------------------------------===// + +This was noticed in the entryblock for grokdeclarator in 403.gcc: + + %tmp = icmp eq i32 %decl_context, 4 + %decl_context_addr.0 = select i1 %tmp, i32 3, i32 %decl_context + %tmp1 = icmp eq i32 %decl_context_addr.0, 1 + %decl_context_addr.1 = select i1 %tmp1, i32 0, i32 %decl_context_addr.0 + +tmp1 should be simplified to something like: + (!tmp || decl_context == 1) + +This allows recursive simplifications, tmp1 is used all over the place in +the function, e.g. by: + + %tmp23 = icmp eq i32 %decl_context_addr.1, 0 ; [#uses=1] + %tmp24 = xor i1 %tmp1, true ; [#uses=1] + %or.cond8 = and i1 %tmp23, %tmp24 ; [#uses=1] + +later. + +//===---------------------------------------------------------------------===// + +Store sinking: This code: + +void f (int n, int *cond, int *res) { + int i; + *res = 0; + for (i = 0; i < n; i++) + if (*cond) + *res ^= 234; /* (*) */ +} + +On this function GVN hoists the fully redundant value of *res, but nothing +moves the store out. This gives us this code: + +bb: ; preds = %bb2, %entry + %.rle = phi i32 [ 0, %entry ], [ %.rle6, %bb2 ] + %i.05 = phi i32 [ 0, %entry ], [ %indvar.next, %bb2 ] + %1 = load i32* %cond, align 4 + %2 = icmp eq i32 %1, 0 + br i1 %2, label %bb2, label %bb1 + +bb1: ; preds = %bb + %3 = xor i32 %.rle, 234 + store i32 %3, i32* %res, align 4 + br label %bb2 + +bb2: ; preds = %bb, %bb1 + %.rle6 = phi i32 [ %3, %bb1 ], [ %.rle, %bb ] + %indvar.next = add i32 %i.05, 1 + %exitcond = icmp eq i32 %indvar.next, %n + br i1 %exitcond, label %return, label %bb + +DSE should sink partially dead stores to get the store out of the loop. + +Here's another partial dead case: +http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12395 + +//===---------------------------------------------------------------------===// + +Scalar PRE hoists the mul in the common block up to the else: + +int test (int a, int b, int c, int g) { + int d, e; + if (a) + d = b * c; + else + d = b - c; + e = b * c + g; + return d + e; +} + +It would be better to do the mul once to reduce codesize above the if. +This is GCC PR38204. + +//===---------------------------------------------------------------------===// + +GCC PR37810 is an interesting case where we should sink load/store reload +into the if block and outside the loop, so we don't reload/store it on the +non-call path. + +for () { + *P += 1; + if () + call(); + else + ... +-> +tmp = *P +for () { + tmp += 1; + if () { + *P = tmp; + call(); + tmp = *P; + } else ... +} +*P = tmp; + +We now hoist the reload after the call (Transforms/GVN/lpre-call-wrap.ll), but +we don't sink the store. We need partially dead store sinking. + +//===---------------------------------------------------------------------===// + +[PHI TRANSLATE GEPs] + +GCC PR37166: Sinking of loads prevents SROA'ing the "g" struct on the stack +leading to excess stack traffic. This could be handled by GVN with some crazy +symbolic phi translation. The code we get looks like (g is on the stack): + +bb2: ; preds = %bb1 +.. + %9 = getelementptr %struct.f* %g, i32 0, i32 0 + store i32 %8, i32* %9, align bel %bb3 + +bb3: ; preds = %bb1, %bb2, %bb + %c_addr.0 = phi %struct.f* [ %g, %bb2 ], [ %c, %bb ], [ %c, %bb1 ] + %b_addr.0 = phi %struct.f* [ %b, %bb2 ], [ %g, %bb ], [ %b, %bb1 ] + %10 = getelementptr %struct.f* %c_addr.0, i32 0, i32 0 + %11 = load i32* %10, align 4 + +%11 is fully redundant, an in BB2 it should have the value %8. + +GCC PR33344 is a similar case. + +//===---------------------------------------------------------------------===// + +There are many load PRE testcases in testsuite/gcc.dg/tree-ssa/loadpre* in the +GCC testsuite. There are many pre testcases as ssa-pre-*.c + +//===---------------------------------------------------------------------===// + +There are some interesting cases in testsuite/gcc.dg/tree-ssa/pred-comm* in the +GCC testsuite. For example, predcom-1.c is: + + for (i = 2; i < 1000; i++) + fib[i] = (fib[i-1] + fib[i - 2]) & 0xffff; + +which compiles into: + +bb1: ; preds = %bb1, %bb1.thread + %indvar = phi i32 [ 0, %bb1.thread ], [ %0, %bb1 ] + %i.0.reg2mem.0 = add i32 %indvar, 2 + %0 = add i32 %indvar, 1 ; [#uses=3] + %1 = getelementptr [1000 x i32]* @fib, i32 0, i32 %0 + %2 = load i32* %1, align 4 ; [#uses=1] + %3 = getelementptr [1000 x i32]* @fib, i32 0, i32 %indvar + %4 = load i32* %3, align 4 ; [#uses=1] + %5 = add i32 %4, %2 ; [#uses=1] + %6 = and i32 %5, 65535 ; [#uses=1] + %7 = getelementptr [1000 x i32]* @fib, i32 0, i32 %i.0.reg2mem.0 + store i32 %6, i32* %7, align 4 + %exitcond = icmp eq i32 %0, 998 ; [#uses=1] + br i1 %exitcond, label %return, label %bb1 + +This is basically: + LOAD fib[i+1] + LOAD fib[i] + STORE fib[i+2] + +instead of handling this as a loop or other xform, all we'd need to do is teach +load PRE to phi translate the %0 add (i+1) into the predecessor as (i'+1+1) = +(i'+2) (where i' is the previous iteration of i). This would find the store +which feeds it. + +predcom-2.c is apparently the same as predcom-1.c +predcom-3.c is very similar but needs loads feeding each other instead of +store->load. +predcom-4.c seems the same as the rest. + + +//===---------------------------------------------------------------------===// + +Other simple load PRE cases: +http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35287 [LPRE crit edge splitting] + +http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34677 (licm does this, LPRE crit edge) + llvm-gcc t2.c -S -o - -O0 -emit-llvm | llvm-as | opt -mem2reg -simplifycfg -gvn | llvm-dis + +//===---------------------------------------------------------------------===// + +Type based alias analysis: +http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14705 + +//===---------------------------------------------------------------------===// + +When GVN/PRE finds a store of float* to a must aliases pointer when expecting +an int*, it should turn it into a bitcast. This is a nice generalization of +the SROA hack that would apply to other cases, e.g.: + +int foo(int C, int *P, float X) { + if (C) { + bar(); + *P = 42; + } else + *(float*)P = X; + + return *P; +} + + +One example (that requires crazy phi translation) is: +http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16799 [BITCAST PHI TRANS] + +//===---------------------------------------------------------------------===// + +A/B get pinned to the stack because we turn an if/then into a select instead +of PRE'ing the load/store. This may be fixable in instcombine: +http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37892 + + + +Interesting missed case because of control flow flattening (should be 2 loads): +http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26629 +With: llvm-gcc t2.c -S -o - -O0 -emit-llvm | llvm-as | + opt -mem2reg -gvn -instcombine | llvm-dis +we miss it because we need 1) GEP PHI TRAN, 2) CRIT EDGE 3) MULTIPLE DIFFERENT +VALS PRODUCED BY ONE BLOCK OVER DIFFERENT PATHS + +//===---------------------------------------------------------------------===// + +http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19633 +We could eliminate the branch condition here, loading from null is undefined: + +struct S { int w, x, y, z; }; +struct T { int r; struct S s; }; +void bar (struct S, int); +void foo (int a, struct T b) +{ + struct S *c = 0; + if (a) + c = &b.s; + bar (*c, a); +} + +//===---------------------------------------------------------------------===// + +simplifylibcalls should do several optimizations for strspn/strcspn: + +strcspn(x, "") -> strlen(x) +strcspn("", x) -> 0 +strspn("", x) -> 0 +strspn(x, "") -> strlen(x) +strspn(x, "a") -> strchr(x, 'a')-x + +strcspn(x, "a") -> inlined loop for up to 3 letters (similarly for strspn): + +size_t __strcspn_c3 (__const char *__s, int __reject1, int __reject2, + int __reject3) { + register size_t __result = 0; + while (__s[__result] != '\0' && __s[__result] != __reject1 && + __s[__result] != __reject2 && __s[__result] != __reject3) + ++__result; + return __result; +} + +This should turn into a switch on the character. See PR3253 for some notes on +codegen. + +456.hmmer apparently uses strcspn and strspn a lot. 471.omnetpp uses strspn. + +//===---------------------------------------------------------------------===// + +"gas" uses this idiom: + else if (strchr ("+-/*%|&^:[]()~", *intel_parser.op_string)) +.. + else if (strchr ("<>", *intel_parser.op_string) + +Those should be turned into a switch. + +//===---------------------------------------------------------------------===// + +252.eon contains this interesting code: + + %3072 = getelementptr [100 x i8]* %tempString, i32 0, i32 0 + %3073 = call i8* @strcpy(i8* %3072, i8* %3071) nounwind + %strlen = call i32 @strlen(i8* %3072) ; uses = 1 + %endptr = getelementptr [100 x i8]* %tempString, i32 0, i32 %strlen + call void @llvm.memcpy.i32(i8* %endptr, + i8* getelementptr ([5 x i8]* @"\01LC42", i32 0, i32 0), i32 5, i32 1) + %3074 = call i32 @strlen(i8* %endptr) nounwind readonly + +This is interesting for a couple reasons. First, in this: + + %3073 = call i8* @strcpy(i8* %3072, i8* %3071) nounwind + %strlen = call i32 @strlen(i8* %3072) + +The strlen could be replaced with: %strlen = sub %3072, %3073, because the +strcpy call returns a pointer to the end of the string. Based on that, the +endptr GEP just becomes equal to 3073, which eliminates a strlen call and GEP. + +Second, the memcpy+strlen strlen can be replaced with: + + %3074 = call i32 @strlen([5 x i8]* @"\01LC42") nounwind readonly + +Because the destination was just copied into the specified memory buffer. This, +in turn, can be constant folded to "4". + +In other code, it contains: + + %endptr6978 = bitcast i8* %endptr69 to i32* + store i32 7107374, i32* %endptr6978, align 1 + %3167 = call i32 @strlen(i8* %endptr69) nounwind readonly + +Which could also be constant folded. Whatever is producing this should probably +be fixed to leave this as a memcpy from a string. + +Further, eon also has an interesting partially redundant strlen call: + +bb8: ; preds = %_ZN18eonImageCalculatorC1Ev.exit + %682 = getelementptr i8** %argv, i32 6 ; [#uses=2] + %683 = load i8** %682, align 4 ; [#uses=4] + %684 = load i8* %683, align 1 ; [#uses=1] + %685 = icmp eq i8 %684, 0 ; [#uses=1] + br i1 %685, label %bb10, label %bb9 + +bb9: ; preds = %bb8 + %686 = call i32 @strlen(i8* %683) nounwind readonly + %687 = icmp ugt i32 %686, 254 ; [#uses=1] + br i1 %687, label %bb10, label %bb11 + +bb10: ; preds = %bb9, %bb8 + %688 = call i32 @strlen(i8* %683) nounwind readonly + +This could be eliminated by doing the strlen once in bb8, saving code size and +improving perf on the bb8->9->10 path. + +//===---------------------------------------------------------------------===// + +I see an interesting fully redundant call to strlen left in 186.crafty:InputMove +which looks like: + %movetext11 = getelementptr [128 x i8]* %movetext, i32 0, i32 0 + + +bb62: ; preds = %bb55, %bb53 + %promote.0 = phi i32 [ %169, %bb55 ], [ 0, %bb53 ] + %171 = call i32 @strlen(i8* %movetext11) nounwind readonly align 1 + %172 = add i32 %171, -1 ; [#uses=1] + %173 = getelementptr [128 x i8]* %movetext, i32 0, i32 %172 + +... no stores ... + br i1 %or.cond, label %bb65, label %bb72 + +bb65: ; preds = %bb62 + store i8 0, i8* %173, align 1 + br label %bb72 + +bb72: ; preds = %bb65, %bb62 + %trank.1 = phi i32 [ %176, %bb65 ], [ -1, %bb62 ] + %177 = call i32 @strlen(i8* %movetext11) nounwind readonly align 1 + +Note that on the bb62->bb72 path, that the %177 strlen call is partially +redundant with the %171 call. At worst, we could shove the %177 strlen call +up into the bb65 block moving it out of the bb62->bb72 path. However, note +that bb65 stores to the string, zeroing out the last byte. This means that on +that path the value of %177 is actually just %171-1. A sub is cheaper than a +strlen! + +This pattern repeats several times, basically doing: + + A = strlen(P); + P[A-1] = 0; + B = strlen(P); + where it is "obvious" that B = A-1. + +//===---------------------------------------------------------------------===// + +186.crafty contains this interesting pattern: + +%77 = call i8* @strstr(i8* getelementptr ([6 x i8]* @"\01LC5", i32 0, i32 0), + i8* %30) +%phitmp648 = icmp eq i8* %77, getelementptr ([6 x i8]* @"\01LC5", i32 0, i32 0) +br i1 %phitmp648, label %bb70, label %bb76 + +bb70: ; preds = %OptionMatch.exit91, %bb69 + %78 = call i32 @strlen(i8* %30) nounwind readonly align 1 ; [#uses=1] + +This is basically: + cststr = "abcdef"; + if (strstr(cststr, P) == cststr) { + x = strlen(P); + ... + +The strstr call would be significantly cheaper written as: + +cststr = "abcdef"; +if (memcmp(P, str, strlen(P))) + x = strlen(P); + +This is memcmp+strlen instead of strstr. This also makes the strlen fully +redundant. + +//===---------------------------------------------------------------------===// + +186.crafty also contains this code: + +%1906 = call i32 @strlen(i8* getelementptr ([32 x i8]* @pgn_event, i32 0,i32 0)) +%1907 = getelementptr [32 x i8]* @pgn_event, i32 0, i32 %1906 +%1908 = call i8* @strcpy(i8* %1907, i8* %1905) nounwind align 1 +%1909 = call i32 @strlen(i8* getelementptr ([32 x i8]* @pgn_event, i32 0,i32 0)) +%1910 = getelementptr [32 x i8]* @pgn_event, i32 0, i32 %1909 + +The last strlen is computable as 1908-@pgn_event, which means 1910=1908. + +//===---------------------------------------------------------------------===// + +186.crafty has this interesting pattern with the "out.4543" variable: + +call void @llvm.memcpy.i32( + i8* getelementptr ([10 x i8]* @out.4543, i32 0, i32 0), + i8* getelementptr ([7 x i8]* @"\01LC28700", i32 0, i32 0), i32 7, i32 1) +%101 = call@printf(i8* ... @out.4543, i32 0, i32 0)) nounwind + +It is basically doing: + + memcpy(globalarray, "string"); + printf(..., globalarray); + +Anyway, by knowing that printf just reads the memory and forward substituting +the string directly into the printf, this eliminates reads from globalarray. +Since this pattern occurs frequently in crafty (due to the "DisplayTime" and +other similar functions) there are many stores to "out". Once all the printfs +stop using "out", all that is left is the memcpy's into it. This should allow +globalopt to remove the "stored only" global. + +//===---------------------------------------------------------------------===// + +This code: + +define inreg i32 @foo(i8* inreg %p) nounwind { + %tmp0 = load i8* %p + %tmp1 = ashr i8 %tmp0, 5 + %tmp2 = sext i8 %tmp1 to i32 + ret i32 %tmp2 +} + +could be dagcombine'd to a sign-extending load with a shift. +For example, on x86 this currently gets this: + + movb (%eax), %al + sarb $5, %al + movsbl %al, %eax + +while it could get this: + + movsbl (%eax), %eax + sarl $5, %eax + +//===---------------------------------------------------------------------===// + +GCC PR31029: + +int test(int x) { return 1-x == x; } // --> return false +int test2(int x) { return 2-x == x; } // --> return x == 1 ? + +Always foldable for odd constants, what is the rule for even? + +//===---------------------------------------------------------------------===// + +PR 3381: GEP to field of size 0 inside a struct could be turned into GEP +for next field in struct (which is at same address). + +For example: store of float into { {{}}, float } could be turned into a store to +the float directly. + +//===---------------------------------------------------------------------===// + +#include +double foo(double a) { return sin(a); } + +This compiles into this on x86-64 Linux: +foo: + subq $8, %rsp + call sin + addq $8, %rsp + ret +vs: + +foo: + jmp sin + +//===---------------------------------------------------------------------===// + +Instcombine should replace the load with a constant in: + + static const char x[4] = {'a', 'b', 'c', 'd'}; + + unsigned int y(void) { + return *(unsigned int *)x; + } + +It currently only does this transformation when the size of the constant +is the same as the size of the integer (so, try x[5]) and the last byte +is a null (making it a C string). There's no need for these restrictions. + +//===---------------------------------------------------------------------===// +