diff options
Diffstat (limited to 'lib/Target/README.txt')
| -rw-r--r-- | lib/Target/README.txt | 115 |
1 files changed, 62 insertions, 53 deletions
diff --git a/lib/Target/README.txt b/lib/Target/README.txt index 2d8a687ebebb5..e1772c2ead0eb 100644 --- a/lib/Target/README.txt +++ b/lib/Target/README.txt @@ -801,8 +801,21 @@ void bar(unsigned n) { true(); } -I think this basically amounts to a dag combine to simplify comparisons against -multiply hi's into a comparison against the mullo. +This is equivalent to the following, where 2863311531 is the multiplicative +inverse of 3, and 1431655766 is ((2^32)-1)/3+1: +void bar(unsigned n) { + if (n * 2863311531U < 1431655766U) + true(); +} + +The same transformation can work with an even modulo with the addition of a +rotate: rotate the result of the multiply to the right by the number of bits +which need to be zero for the condition to be true, and shrink the compare RHS +by the same amount. Unless the target supports rotates, though, that +transformation probably isn't worthwhile. + +The transformation can also easily be made to work with non-zero equality +comparisons: just transform, for example, "n % 3 == 1" to "(n-1) % 3 == 0". //===---------------------------------------------------------------------===// @@ -823,20 +836,6 @@ int main() { //===---------------------------------------------------------------------===// -Instcombine will merge comparisons like (x >= 10) && (x < 20) by producing (x - -10) u< 10, but only when the comparisons have matching sign. - -This could be converted with a similiar technique. (PR1941) - -define i1 @test(i8 %x) { - %A = icmp uge i8 %x, 5 - %B = icmp slt i8 %x, 20 - %C = and i1 %A, %B - ret i1 %C -} - -//===---------------------------------------------------------------------===// - These functions perform the same computation, but produce different assembly. define i8 @select(i8 %x) readnone nounwind { @@ -884,18 +883,6 @@ The expression should optimize to something like //===---------------------------------------------------------------------===// -From GCC Bug 15241: -unsigned int -foo (unsigned int a, unsigned int b) -{ - if (a <= 7 && b <= 7) - baz (); -} -Should combine to "(a|b) <= 7". Currently not optimized with "clang --emit-llvm-bc | opt -std-compile-opts". - -//===---------------------------------------------------------------------===// - From GCC Bug 3756: int pn (int n) @@ -907,19 +894,6 @@ Should combine to (n >> 31) | 1. Currently not optimized with "clang //===---------------------------------------------------------------------===// -From GCC Bug 28685: -int test(int a, int b) -{ - int lt = a < b; - int eq = a == b; - - return (lt || eq); -} -Should combine to "a <= b". Currently not optimized with "clang --emit-llvm-bc | opt -std-compile-opts | llc". - -//===---------------------------------------------------------------------===// - void a(int variable) { if (variable == 4 || variable == 6) @@ -993,12 +967,6 @@ Should combine to 0. Currently not optimized with "clang //===---------------------------------------------------------------------===// -int a(unsigned char* b) {return *b > 99;} -There's an unnecessary zext in the generated code 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". @@ -1011,12 +979,6 @@ Should combine to "x | (y & 3)". Currently not optimized with "clang //===---------------------------------------------------------------------===// -unsigned a(unsigned a) {return ((a | 1) & 3) | (a & -4);} -Should combine to "a | 1". 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". @@ -1704,3 +1666,50 @@ need all but the bottom two bits from %A, and if we gave that mask to SDB it would delete the or instruction for us. //===---------------------------------------------------------------------===// + +FunctionAttrs is not marking this function as readnone (just readonly): +$ clang t.c -emit-llvm -S -o - -O0 | opt -mem2reg -S -functionattrs + +int t(int a, int b, int c) { + int *p; + if (a) + p = &a; + else + p = &c; + return *p; +} + +This is because we codegen this to: + +define i32 @t(i32 %a, i32 %b, i32 %c) nounwind readonly ssp { +entry: + %a.addr = alloca i32 ; <i32*> [#uses=3] + %c.addr = alloca i32 ; <i32*> [#uses=2] +... + +if.end: + %p.0 = phi i32* [ %a.addr, %if.then ], [ %c.addr, %if.else ] + %tmp2 = load i32* %p.0 ; <i32> [#uses=1] + ret i32 %tmp2 +} + +And functionattrs doesn't realize that the p.0 load points to function local +memory. + +Also, functionattrs doesn't know about memcpy/memset. This function should be +marked readnone, since it only twiddles local memory, but functionattrs doesn't +handle memset/memcpy/memmove aggressively: + +struct X { int *p; int *q; }; +int foo() { + int i = 0, j = 1; + struct X x, y; + int **p; + y.p = &i; + x.q = &j; + p = __builtin_memcpy (&x, &y, sizeof (int *)); + return **p; +} + +//===---------------------------------------------------------------------===// + |