[ValueTracking] determine sign of 0.0 from select when matching min/max FP

In PR39475:
https://bugs.llvm.org/show_bug.cgi?id=39475
..we may fail to recognize/simplify fabs() in some cases because we do not 
canonicalize fcmp with a -0.0 operand.

Adding that canonicalization can cause regressions on min/max FP tests, so 
that's this patch: for the purpose of determining whether something is min/max, 
let the value returned by the select determine how we treat a 0.0 operand in the fcmp.

This patch doesn't actually change the -0.0 to +0.0. It just changes the analysis, so 
we don't fail to recognize equivalent min/max patterns that only differ in the 
signbit of 0.0.

Differential Revision: https://reviews.llvm.org/D54001

llvm-svn: 346097

1 files changed, 15 insertions, 23 deletions

diff --git a/llvm/test/Transforms/InstCombine/minmax-fp.ll b/llvm/test/Transforms/InstCombine/minmax-fp.ll
index 292e50eb1f9..7bf8f57d4e8 100644
--- a/llvm/test/Transforms/InstCombine/minmax-fp.ll
+++ b/llvm/test/Transforms/InstCombine/minmax-fp.ll
@@ -57,16 +57,15 @@ define double @t5(float %a) {
   ret double %3
 }
 
-; TODO:
-; From IEEE754: "Comparisons shall ignore the sign of zero (so +0 = −0)."
+; From IEEE754: "Comparisons shall ignore the sign of zero (so +0 = -0)."
 ; So the compare constant may be treated as +0.0, and we sink the fpext.
 
 define double @t6(float %a) {
 ; CHECK-LABEL: @t6(
-; CHECK-NEXT:    [[TMP1:%.*]] = fcmp ult float [[A:%.*]], -0.000000e+00
-; CHECK-NEXT:    [[TMP2:%.*]] = fpext float [[A]] to double
-; CHECK-NEXT:    [[TMP3:%.*]] = select i1 [[TMP1]], double [[TMP2]], double 0.000000e+00
-; CHECK-NEXT:    ret double [[TMP3]]
+; CHECK-NEXT:    [[DOTINV:%.*]] = fcmp oge float [[A:%.*]], 0.000000e+00
+; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[DOTINV]], float 0.000000e+00, float [[A]]
+; CHECK-NEXT:    [[TMP2:%.*]] = fpext float [[TMP1]] to double
+; CHECK-NEXT:    ret double [[TMP2]]
 ;
   %1 = fcmp ult float %a, -0.0
   %2 = fpext float %a to double
@@ -74,16 +73,15 @@ define double @t6(float %a) {
   ret double %3
 }
 
-; TODO:
-; From IEEE754: "Comparisons shall ignore the sign of zero (so +0 = −0)."
+; From IEEE754: "Comparisons shall ignore the sign of zero (so +0 = -0)."
 ; So the compare constant may be treated as -0.0, and we sink the fpext.
 
 define double @t7(float %a) {
 ; CHECK-LABEL: @t7(
-; CHECK-NEXT:    [[TMP1:%.*]] = fcmp ult float [[A:%.*]], 0.000000e+00
-; CHECK-NEXT:    [[TMP2:%.*]] = fpext float [[A]] to double
-; CHECK-NEXT:    [[TMP3:%.*]] = select i1 [[TMP1]], double [[TMP2]], double -0.000000e+00
-; CHECK-NEXT:    ret double [[TMP3]]
+; CHECK-NEXT:    [[DOTINV:%.*]] = fcmp oge float [[A:%.*]], -0.000000e+00
+; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[DOTINV]], float -0.000000e+00, float [[A]]
+; CHECK-NEXT:    [[TMP2:%.*]] = fpext float [[TMP1]] to double
+; CHECK-NEXT:    ret double [[TMP2]]
 ;
   %1 = fcmp ult float %a, 0.0
   %2 = fpext float %a to double
@@ -91,15 +89,12 @@ define double @t7(float %a) {
   ret double %3
 }
 
-; TODO:
 ; min(min(x, 0.0), 0.0) --> min(x, 0.0)
 
 define float @fmin_fmin_zero_mismatch(float %x) {
 ; CHECK-LABEL: @fmin_fmin_zero_mismatch(
-; CHECK-NEXT:    [[CMP1:%.*]] = fcmp olt float [[X:%.*]], -0.000000e+00
-; CHECK-NEXT:    [[MIN1:%.*]] = select i1 [[CMP1]], float [[X]], float 0.000000e+00
-; CHECK-NEXT:    [[CMP2:%.*]] = fcmp olt float [[MIN1]], 0.000000e+00
-; CHECK-NEXT:    [[MIN2:%.*]] = select i1 [[CMP2]], float [[MIN1]], float 0.000000e+00
+; CHECK-NEXT:    [[TMP1:%.*]] = fcmp olt float [[X:%.*]], 0.000000e+00
+; CHECK-NEXT:    [[MIN2:%.*]] = select i1 [[TMP1]], float [[X]], float 0.000000e+00
 ; CHECK-NEXT:    ret float [[MIN2]]
 ;
   %cmp1 = fcmp olt float %x, -0.0
@@ -109,16 +104,13 @@ define float @fmin_fmin_zero_mismatch(float %x) {
   ret float %min2
 }
 
-; TODO:
 ; max(max(x, -0.0), -0.0) --> max(x, -0.0)
 
 define float @fmax_fmax_zero_mismatch(float %x) {
 ; CHECK-LABEL: @fmax_fmax_zero_mismatch(
-; CHECK-NEXT:    [[CMP1:%.*]] = fcmp ogt float [[X:%.*]], 0.000000e+00
-; CHECK-NEXT:    [[MAX1:%.*]] = select i1 [[CMP1]], float [[X]], float -0.000000e+00
-; CHECK-NEXT:    [[CMP2:%.*]] = fcmp olt float [[MAX1]], 0.000000e+00
-; CHECK-NEXT:    [[MAX2:%.*]] = select i1 [[CMP2]], float -0.000000e+00, float [[MAX1]]
-; CHECK-NEXT:    ret float [[MAX2]]
+; CHECK-NEXT:    [[TMP1:%.*]] = fcmp ogt float [[X:%.*]], -0.000000e+00
+; CHECK-NEXT:    [[MAX11:%.*]] = select i1 [[TMP1]], float [[X]], float -0.000000e+00
+; CHECK-NEXT:    ret float [[MAX11]]
 ;
   %cmp1 = fcmp ogt float %x, 0.0
   %max1 = select i1 %cmp1, float %x, float -0.0