[InstCombine] Canonicalize (uitofp x) -> (sitofp x) if x >= 0

Generally we support more transforms peeking through `sitofp`

Proofs: https://alive2.llvm.org/ce/z/9W4VFm

Author: goldsteinn

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -1929,7 +1929,11 @@ Instruction *InstCombinerImpl::visitFPToSI(FPToSIInst &FI) {
 }
 
 Instruction *InstCombinerImpl::visitUIToFP(CastInst &CI) {
-  return commonCastTransforms(CI);
+  if (Instruction *R = commonCastTransforms(CI))
+    return R;
+  if (isKnownNonNegative(CI.getOperand(0), SQ))
+    return new SIToFPInst(CI.getOperand(0), CI.getType());
+  return nullptr;
 }
 
 Instruction *InstCombinerImpl::visitSIToFP(CastInst &CI) {

llvm/test/Transforms/InstCombine/add-sitofp.ll

@@ -67,9 +67,8 @@ define double @test_2_uitofp(i32 %a, i32 %b) {
 ; CHECK-LABEL: @test_2_uitofp(
 ; CHECK-NEXT:    [[A_AND:%.*]] = and i32 [[A:%.*]], 1073741823
 ; CHECK-NEXT:    [[B_AND:%.*]] = and i32 [[B:%.*]], 1073741823
-; CHECK-NEXT:    [[A_AND_FP:%.*]] = uitofp i32 [[A_AND]] to double
-; CHECK-NEXT:    [[B_AND_FP:%.*]] = uitofp i32 [[B_AND]] to double
-; CHECK-NEXT:    [[RES:%.*]] = fadd double [[A_AND_FP]], [[B_AND_FP]]
+; CHECK-NEXT:    [[ADDCONV:%.*]] = add nuw nsw i32 [[A_AND]], [[B_AND]]
+; CHECK-NEXT:    [[RES:%.*]] = sitofp i32 [[ADDCONV]] to double
 ; CHECK-NEXT:    ret double [[RES]]
 ;
   ; Drop two highest bits to guarantee that %a + %b doesn't overflow

llvm/test/Transforms/InstCombine/clamp-to-minmax.ll

@@ -472,7 +472,7 @@ define float @ui32_clamp_and_cast_to_float(i32 %x) {
 ; CHECK-LABEL: @ui32_clamp_and_cast_to_float(
 ; CHECK-NEXT:    [[LO_CMP:%.*]] = icmp eq i32 [[X:%.*]], 0
 ; CHECK-NEXT:    [[MIN1:%.*]] = call i32 @llvm.umin.i32(i32 [[X]], i32 255)
-; CHECK-NEXT:    [[MIN:%.*]] = uitofp i32 [[MIN1]] to float
+; CHECK-NEXT:    [[MIN:%.*]] = sitofp i32 [[MIN1]] to float
 ; CHECK-NEXT:    [[R:%.*]] = select i1 [[LO_CMP]], float 1.000000e+00, float [[MIN]]
 ; CHECK-NEXT:    ret float [[R]]
 ;
@@ -488,7 +488,7 @@ define float @ui64_clamp_and_cast_to_float(i64 %x) {
 ; CHECK-LABEL: @ui64_clamp_and_cast_to_float(
 ; CHECK-NEXT:    [[LO_CMP:%.*]] = icmp eq i64 [[X:%.*]], 0
 ; CHECK-NEXT:    [[MIN1:%.*]] = call i64 @llvm.umin.i64(i64 [[X]], i64 255)
-; CHECK-NEXT:    [[MIN:%.*]] = uitofp i64 [[MIN1]] to float
+; CHECK-NEXT:    [[MIN:%.*]] = sitofp i64 [[MIN1]] to float
 ; CHECK-NEXT:    [[R:%.*]] = select i1 [[LO_CMP]], float 1.000000e+00, float [[MIN]]
 ; CHECK-NEXT:    ret float [[R]]
 ;

llvm/test/Transforms/InstCombine/fpcast.ll

@@ -266,7 +266,7 @@ define half @uint_to_fptrunc(i32 %x) {
 define half @masked_uint_to_fptrunc1(i32 %x) {
 ; CHECK-LABEL: @masked_uint_to_fptrunc1(
 ; CHECK-NEXT:    [[M:%.*]] = and i32 [[X:%.*]], 16777215
-; CHECK-NEXT:    [[R:%.*]] = uitofp i32 [[M]] to half
+; CHECK-NEXT:    [[R:%.*]] = sitofp i32 [[M]] to half
 ; CHECK-NEXT:    ret half [[R]]
 ;
   %m = and i32 %x, 16777215
@@ -278,7 +278,7 @@ define half @masked_uint_to_fptrunc1(i32 %x) {
 define half @masked_uint_to_fptrunc2(i32 %x) {
 ; CHECK-LABEL: @masked_uint_to_fptrunc2(
 ; CHECK-NEXT:    [[M:%.*]] = lshr i32 [[X:%.*]], 8
-; CHECK-NEXT:    [[R:%.*]] = uitofp i32 [[M]] to half
+; CHECK-NEXT:    [[R:%.*]] = sitofp i32 [[M]] to half
 ; CHECK-NEXT:    ret half [[R]]
 ;
   %m = lshr i32 %x, 8
@@ -290,7 +290,7 @@ define half @masked_uint_to_fptrunc2(i32 %x) {
 define half @masked_uint_to_fptrunc3(i32 %x) {
 ; CHECK-LABEL: @masked_uint_to_fptrunc3(
 ; CHECK-NEXT:    [[M:%.*]] = lshr i32 [[X:%.*]], 7
-; CHECK-NEXT:    [[F:%.*]] = uitofp i32 [[M]] to float
+; CHECK-NEXT:    [[F:%.*]] = sitofp i32 [[M]] to float
 ; CHECK-NEXT:    [[R:%.*]] = fptrunc float [[F]] to half
 ; CHECK-NEXT:    ret half [[R]]
 ;
@@ -314,7 +314,7 @@ define double @uint_to_fpext(i32 %x) {
 define double @masked_uint_to_fpext1(i32 %x) {
 ; CHECK-LABEL: @masked_uint_to_fpext1(
 ; CHECK-NEXT:    [[M:%.*]] = and i32 [[X:%.*]], 16777215
-; CHECK-NEXT:    [[R:%.*]] = uitofp i32 [[M]] to double
+; CHECK-NEXT:    [[R:%.*]] = sitofp i32 [[M]] to double
 ; CHECK-NEXT:    ret double [[R]]
 ;
   %m = and i32 %x, 16777215
@@ -326,7 +326,7 @@ define double @masked_uint_to_fpext1(i32 %x) {
 define double @masked_uint_to_fpext2(i32 %x) {
 ; CHECK-LABEL: @masked_uint_to_fpext2(
 ; CHECK-NEXT:    [[M:%.*]] = lshr i32 [[X:%.*]], 8
-; CHECK-NEXT:    [[R:%.*]] = uitofp i32 [[M]] to double
+; CHECK-NEXT:    [[R:%.*]] = sitofp i32 [[M]] to double
 ; CHECK-NEXT:    ret double [[R]]
 ;
   %m = lshr i32 %x, 8
@@ -338,7 +338,7 @@ define double @masked_uint_to_fpext2(i32 %x) {
 define double @masked_uint_to_fpext3(i32 %x) {
 ; CHECK-LABEL: @masked_uint_to_fpext3(
 ; CHECK-NEXT:    [[M:%.*]] = lshr i32 [[X:%.*]], 7
-; CHECK-NEXT:    [[F:%.*]] = uitofp i32 [[M]] to float
+; CHECK-NEXT:    [[F:%.*]] = sitofp i32 [[M]] to float
 ; CHECK-NEXT:    [[R:%.*]] = fpext float [[F]] to double
 ; CHECK-NEXT:    ret double [[R]]
 ;

llvm/test/Transforms/InstCombine/sitofp.ll

@@ -256,7 +256,7 @@ define i25 @consider_lowbits_masked_input(i25 %A) {
 define i32 @overflow_masked_input(i32 %A) {
 ; CHECK-LABEL: @overflow_masked_input(
 ; CHECK-NEXT:    [[M:%.*]] = and i32 [[A:%.*]], 16777217
-; CHECK-NEXT:    [[B:%.*]] = uitofp i32 [[M]] to float
+; CHECK-NEXT:    [[B:%.*]] = sitofp i32 [[M]] to float
 ; CHECK-NEXT:    [[C:%.*]] = fptoui float [[B]] to i32
 ; CHECK-NEXT:    ret i32 [[C]]
 ;